what file to put float property and properties class blender 2.8
A clean approach for registration is to provide a register and unregister function in every file that contains classes that need to be registered. Provide a top level register and unregister function in the __init__.py which calls the respective functions of the imported modules.

You can put the property groups in a separate file, they don't have to be in the same file as the panels.

A good reference for this approach is the Cycles add-on that can be found in your Blender install [version]/scripts/addons/cycles or intern/cycles/blender/addon in the repository.